Below are excepts on this question from various Orthodox Christian sources on this question.
I would only like to add that the information below should not be taken as minimizing or excusing the seriousness of doctrinal error and heresy on the human soul, on corporate Christian bodies, and society at large.
When a Protestant hears the claims of the Orthodox, he or she may make two assumptions: (1) that we believe no one outside the Orthodox Church will be saved, and (2) that we believe all Orthodox Christians will be saved. Neither of these is correct. Although membership in the historical Church is the normative means of union with God, and it can be certain that [the fullness] of grace and life of Christ are found within her bosom [if one pursues it with authentic faith], the Church does not make judgments about the salvation of any particular persons. The Orthodox Church follows the teaching of the Apostle Paul, who said, “For what have I to do with judging those also who are outside?”….But those who are outside God judges” (1 Cor. 5:12-13)
From: Know the Faith: A Handbook for Orthodox Christians and Inquirers. Fr. Michael Shanbour, Ancient Faith Publishing, © 2016 Michael Alan Shanbour.
* * *
With reference to the above question, it is particularly instructive to recall the answer once given to an inquirer by [Saint] Theophan the Recluse. The blessed one replied more or less thus:
“You ask, will the heterodox be saved…Why do you worry about them? They have a Savior Who desires the salvation of every human being. He will take care of them. You and I should not be burdened with such a concern. Study yourself and your own sins….”
In attempting to answer this question, it is necessary, first of all, to recall that in His Gospel the Lord Jesus Christ Himself mentions but one state of the human soul which unfailingly leads to perdition—i.e. blasphemy against the Holy Spirit (Matt. 12:1-32). The Holy Spirit is, above all, the Spirit of Truth, as the Savior loved to refer to Him. Accordingly, blasphemy against the Holy Spirit is blasphemy against the Truth, conscious and persistent opposition to it. The same text makes it clear that even blasphemy against the Son of Man—i.e. the Lord Jesus Christ, the incarnate Son of God Himself may be forgiven men, as it may be uttered in error or in ignorance and, subsequently may be covered by conversion and repentance (an example of such a converted and repentant blasphemer is the Apostle Paul. (See Acts 26:11 and I Tim. 1:13.) If, however, a man opposes the Truth which he clearly apprehends by his reason and, conscience, he becomes blind and commits spiritual suicide, for he thereby likens himself to the devil, who believes in God and dreads Him, yet hates, blasphemes, and opposes Him.
Thus, man’s refusal to accept the Divine Truth and his opposition thereto makes him a son of damnation. Accordingly, in sending His disciples to preach, the Lord told them: “He that believeth and is baptized shall be saved, but he that believeth not shall be damned” (Mk. 16:16), for the latter heard the Lord’s Truth and was called upon to accept it, yet refused, thereby inheriting the damnation of those who “believed not the truth, but had pleasure in unrighteousness” (II Thes. 2:12).
The Holy Orthodox Church is the repository of the divinely revealed Truth in all its fullness and fidelity to apostolic Tradition. Hence, he who leaves the Church, who intentionally and consciously falls away from it, joins the ranks of its opponents and becomes a renegade as regards apostolic Tradition. The Church dreadfully anathematized such renegades, in accordance with the words of the Savior Himself (Matt. 18:17) and of the Apostle Paul (Gal. 1:8-9), threatening them with eternal damnation and calling them to return to the Orthodox fold. It is self evident, however, that sincere Christians who are Roman Catholics, or Lutherans, or members, of other non-Orthodox confessions, cannot be termed renegades or heretics—i.e. those who knowingly pervert the truth…* They have been born and raised and are living according to the creed which they have inherited, just as do the majority of you who are Orthodox; in their lives there has not been a moment of personal and conscious renunciation of Orthodoxy. The Lord, “Who will have all men to be saved” (I Tim. 2:4) and “Who enlightens every man born into the world” (Jn. 1.43), undoubtedly is leading them also towards salvation In His own way.
From: Archimandrite (Metropolitan) Philaret of blessed memory (+1985), http://orthodoxinfo.com/inquirers/metphil_heterodox.aspx
* * *
One of the most difficult things for people to accept about Christianity is the exclusive claim made by Jesus to be God and the only Savior of the world. This claim causes scandal to many, both those (ostensibly) within and those outside the Orthodox Church—a scandal that is simply unavoidable. Outside Christianity, many actively resent any claim that Jesus is God and the only Savior of the world. Yet this is indeed Christ’s claim, from which there is no honorable escape.
…Because of the exclusive claims of the Lord, the Orthodox Church confesses, believes, and proclaims that the only Savior of the world, Jesus Christ, has one Body, His Church…. Postmodern pluralism cannot be honestly reconciled with Jesus’ claims regarding Himself or Orthodoxy’s claims regarding herself. There is but one Truth, and He is Jesus.
…It must be remembered that it is Jesus Christ alone that judges who is or is not saved. The Bible teaches that not all those in the Church will be saved, but some who are never visibly in the Church are nevertheless near and dear to the Lord. (How many times did Samaritan heretics exhibit saving faith in the Gospels?) Jesus is the exclusive Judge of all. On the last and great day, all human beings who have ever lived will be brought before the Lord for the final Judgment. The Creed of Nicea-Constantinople adequately summarizes the entire Tradition when it says of Jesus, He will come again in glory to judge the living and the dead.
… It must be admitted that the exclusive claims regarding Christ and the Church are insufferable when proclaimed by haughty souls who would not recognize the virtue of humility if they fell over it. No Orthodox Christian is justified in boasting or presuming that his or her visible membership in the Church is a guarantee of a place at the marriage supper of the Lamb. As much damage to sensitive souls is probably done by presumptuous arrogance on the part of Orthodox believers as is done by all the anti-Christian postmodernist academicians combined.
… We can say much about those who will be ushered into the marriage supper of the Lamb, but we cannot say everything. Only the Lord has all the facts regarding the saved and the lost. God is Love; He is just and merciful. No one will be unjustly excluded from the Presence of God and the bliss of the day without evening. But there will be many surprises on that day, one of which will be the many excluded from the joy of the Kingdom who considered themselves members in good standing of the Orthodox Church!
… We know some things about where the Church is and who is part of her. Even now we know the names of many people who will be present at the marriage supper of the Lamb [e.g. the Mother of the Lord, St. John the Baptist, the Apostles, etc.]… When it comes to who will not be at the marriage supper of the Lamb, we have a much shorter list of specific names. The horror of any creature’s ultimate rejection of the mercy of God is almost too much to contemplate.
… Judas Iscariot was one of the Twelve. Was the Lord’s call a charade? Arius was a duly ordained senior priest of the Church of Alexandria. Was his ordination flawed? Absolutely not! Judas’ call was genuine and pure; Judas himself was not. Arius’ ordination was faithful and authentic; Arius was not. From the time of Judas to today, mere visible membership in the Church is no guarantee of salvation. It can, however, guarantee a stricter standard of judgment by the Lord, especially for the ordained. The Lord judges the reality, not merely the appearance, of our faith and obedience. He will separate the saved from the lost on the Last Day.
… We are simply not given all the facts regarding the mystery of even our own salvation, much less anyone else’s.
From: Fr. David Tillman, This article is available as a printed booklet from Ancient Faith Publishing, a department of the Antiochian Archdiocese, as part of their popular series of attractive and informative booklets and brochures about the basic teachings of the ancient Orthodox Christian faith. © Ancient Faith Publishing.
* * *
Christ our God may be working in others in ways unknown to us and even to them, to bring them to salvation. And in due time, perhaps not till after death, they may recognize God and accept Christ and be united to His Body the Church-so that they can be saved.
This is in accord with the teaching of Christ. In the parable of the sheep and the goats (Matthew 25), notice that it is the “nations” (v. 32), the nonbelievers, who are being judged (this is obvious, because they are surprised to learn that Christ dwells in the needy), and some of them are welcomed into the “kingdom prepared for [them] from the foundation of the world” (v. 34). Regarding God’s mysterious work outside the Orthodox Church, we have nothing to say. We make no judgments about what God is doing there, or about what happens to the souls of those who are not Orthodox or not Christian on earth. It is all we can do to try to “work out [our] own salvation with fear and trembling” (Philippians 2:12).